This is the first application filed for the present invention.
Not Applicable.
The present invention relates to optical transmission systems for telecommunications, and in particular, to a method and system for adaptive optical amplifier control.
Long distance data transmission in optical networks requires periodic amplification of the optical signals to compensate for attenuation due to the cumulative effects of absorption and scatter in optical fibers. Different types of optical amplifiers can be used for this purpose such as for example, Raman amplifiers, erbium doped fiber amplifiers (EDFA) and variable optical attenuators (VOA). Optical amplifiers can also consist of several types of amplifying elements and thus can be considered hybrid amplifiers. In general, optical amplifiers can be controlled by several different control variables and the performance of the amplifiers can be determined by measuring several different output parameters. A general problem is thus, how to control multiple inputs (the control variables), to achieve desired values of multiple output parameters.
One example of an optical amplifier is the Raman amplifier, which plays an important role in optical communication systems because it permit longer fiber spans. It provides better signal amplification while introducing less noise than a traditional signal amplifier. The Raman amplifier compensates for fiber loss by providing signal gain in every span. It uses the non-linear scattering property of optical fiber known as Stimulated Raman Scattering (SRS) to transfer energy from pump lasers to signal channels.
The Raman effect causes light traveling within a medium, such as an optical fiber, to be amplified by the presence of shorter wavelength light traveling within the same medium. Energy is transferred from the shorter wavelength light to a longer wavelength signal. An exemplary gain spectrum of a silica fiber pumped by a single monochromatic Raman pump is illustrated in FIG. 1a. Multiple Raman pump lasers at different wavelengths can be used to spread this influence over a wider range of longer wavelengths. Backward Raman amplifiers typically use up to twelve (or more) pump lasers to provide amplification for across all of the signal channels of a Dense Wave-Division Multiplexed (DWDM) communications system. FIG. 1b illustrates exemplary gain profiles showing contributions of four Raman pumps to optical signal gain across a range of signal wavelengths. The net Raman gain profile is a result of the superposition of the individual pump profiles as well as the effects of nonlinear Raman interactions between the amplifier pumps and the signals.
One of the problems associated with such arrangements is the difficulty in achieving a desired gain (or other desired output parameter) profile over a range of wavelengths. The relative powers required for each pump changes as the mean gain of the amplifier changes, due to the complex interactions resulting from stimulated Raman scattering (SRS) between the various optical wavelengths in the fiber.
Effective use of a Raman amplifier in a DWDM communications network requires measurement of a desired output parameter (e.g. gain) and Raman pump control to obtain a desired parameter profile. To control a multiple pump Raman amplifier, the measured output parameter should be mapped to individual pump powers. Calculating a theoretical relationship between an output parameter and relative pump powers requires solving a non-linear system of differential equations that describe optical signal propagation and the Raman scattering phenomenon in the fiber, which is a tedious and cumbersome task. There are also practical issues which affect the usefulness and accuracy of using theoretical calculations, such as requiring a priori knowledge of the fiber type, accounting for connection losses, variations in fiber core size and attenuation, etc.
One solution to this problem is described in co-pending U.S. patent application Ser. No. 09/873,389 to Seydnejad et al., wherein Raman pump power levels required to provide a uniform gain across a range of wavelengths in an optical fiber, are determined by a combination of theoretical calculations and empirical measurement. A system of non-linear differential equations is solved for various pump powers and used to build a look-up table relating desired average signal gain to the relative pump powers required to provide a uniform gain (or other desired profile) across a range of wavelengths. The linear relationship between Raman pump power and average signal gain is determined by measuring data signal power levels at specific Raman pump powers. A desired average signal gain is first applied to the linear relationship to determine total power required, and then applied to the look-up table to determine the required relative pump powers. Disadvantages of this method include complexity of calculating theoretical Raman pump powers, dependence on a theoretical model which may not reflect actual Raman amplifier behavior, the reliance on knowledge of the fiber type, Raman gain coefficients, launch power, etc, lack of flexibility, and the possibility of changing conditions affecting the performance of the optical communication system.
Various operating conditions in optical transmission systems can vary over time. Fiber cables and associated connectors can change over time due to vibration, contamination or other causes. Amplifier characteristics such as gain and available power can change over the life of the amplifier due to aging. Signals can change due to component changes or optical properties such as polarization dependent loss (PDL) or nonlinear interactions. Signals can also change when new channels (signal wavelengths) are introduced into an optical transmission system. Also, any degradation or failure of a component in the optical transmission system can change the operating conditions in complex (frequently non-linear) ways.
Accordingly, a method and apparatus for providing adaptive control of an optical amplifier to achieve a desired output, without requiring a priori information of the optical transmission system, remains highly desirable.
An object of the present invention is to provide a method and apparatus for adaptive control of an optical amplifier.
Accordingly, an aspect of the present invention provides a method of controlling an optical amplifier, the method comprises steps of: calculating an error vector indicative of a difference between respective detected values and target values of a parameter of a light beam downstream of the optical amplifier; calculating a sensitivity matrix indicative of a sensitivity of the detected parameter value to incremental changes in a control variable of the optical amplifier; and calculating a predicted optimum value of the control variable using the error vector and the sensitivity matrix.
Another aspect of the present invention provides a system for controlling an optical amplifier. The system comprises an error calculator for calculating an error vector indicative of a difference between respective detected values and target values of a parameter of a light beam downstream of the optical amplifier; a sensitivity matrix calculator for calculating a sensitivity matrix indicative of a sensitivity of the detected parameter value to incremental changes in a control variable of the optical amplifier; and a controller for calculating a predicted optimum value of the control variable using the error vector and the sensitivity matrix.
The optical amplifier may be any of a Raman amplifier; a Erbium Doped Fiber Amplifier (EDFA); or a hybrid of these. In either case, the control variables may conveniently represent an output power of each pump laser of the amplifier, and the sensitivity matrix represented as a function of each pump output power. The target values may be either fixed, or variable, as desired.
In some embodiments of the invention, the predicted optimum value of the control variable is calculated by determining a control variable value that minimizes the error vector. This may be accomplished using a least mean squares technique.
Preferably, calculation of the predicted optimum control variable value and calculation of the error vector are repeated through one or more iterations in order to converge upon a predicted optimum control variable value that is a best estimate of the optimum control variable value. This iterative calculation procedure can be initiated by a predetermined trigger condition (e.g. when the error vector becomes too large) and terminated by a predetermined termination condition (e.g. a maximum number of iterations; the error vector becomes smaller than a predetermined threshold etc.) The sensitivity matrix may be recalculated during each iteration. Alternatively, the sensitivity matrix can be calculated at the start of an optimization run (i.e. upon detection of the trigger condition) and then held constant for subsequent iterations (i.e. until the termination condition is satisfied).
Thus the present invention provides a method and system for controlling an optical amplifier, which dynamically adapts to both configuration and performance changes of the amplifier and the optical communications system within which it is deployed. An error vector is calculated to indicate a difference between respective detected values and target values of a parameter of a light beam downstream of the optical amplifier. A sensitivity matrix indicative of a sensitivity of the detected parameter value to incremental changes in a control variable of the optical amplifier is calculated. A predicted optimum value of the control variable is then calculated using the error vector and the sensitivity matrix. Calculation of the predicted optimum control variable value can be iterative, with the sensitivity matrix calculated either during each iteration, or at the beginning of each optimization run. As a result, optimization of the amplifier control variables is performed based on a sensitivity matrix that accurately reflects the performance of the amplifier.